1. Field of the Invention
The invention is in the field of joints for ductile iron pressure pipe.
2. Description of the Prior Art
In many applications where pipes are joined in telescoping relationship, the smooth or spigot end of one pipe is inserted into the socket end of the engaging pipe. The socket end has an opening large enough to receive the spigot end of the enclosed pipe. A gasket is inserted in the socket end of tile enclosing pipe and prevents leakage of fluid from the joint by forming a seal between the two pipes.
In applications where the fluid such as water for fire mains or water distribution in municipalities is under high pressure, various means are used to prevent separation of the two pipes.
In some instances the pipes are fitted with flanges which are bolted together to prevent separation. This method is costly and time consuming because of the extra steps of inserting and tightening the bolts.
Another method to prevent separation of the pipes is to use blocks or external restraining devices. This method also is expensive and requires careful placement of the blocks.
A still further method is shown in U.S. Pat. Nos. 3,963,298 and 4,229,026 in which a rubber gasket which serves as a fluid seal also has toothed metal segments spaced uniformly around its inner perimeter. The toothed metal segments bite into the outer surface of the spigot of the inserted pipe and prevent withdrawal of the inserted pipe. The metal segments pivot about a bead to allow the spigot end of one pipe to be inserted into the socket end of the other pipe while preventing withdrawal of the inserted pipe under high fluid pressure conditions.
However, a drawback of this arrangement has been the fracture or breaking of the socketed portion of the inserted pipe when the teeth of the metal segments dig too deeply into the outer surface of the inserted pipe. This fracture has been noted when the metal segments rotate too far around the bead.
Copending application Ser. No. 07/971,110 now U.S. Pat. No. 5,295,697 discloses a restraining element having metal segments spaced equidistantly around an exposed face of the restraining element wherein each of the metal segments has a first exposed face with a row of teeth and a single tooth extending outwardly beyond the face of the metal segment opposite said first exposed face. In that application, the preferred embodiment shows a metal segment having three teeth on one exposed face. The operation of the restraining element of the copending application now U.S. Pat. No. 5,295,697 operates so that two of the teeth of the metal segment will grasp the spigot of the enclosed pipe at a preselected tolerance difference between the bell opening and the outside diameter of the spigot. However, in actual practice the difference in the distance between the lower edge of the throat portion of the bell of one pipe and the outer surface of the spigot of the enclosed pipe may vary over a wide range and the invention of the copending application may not completely and adequately compensate for these differences.